The present invention concerns an ultra-thin piezoelectric resonator, and more particularly, a resonator of very small dimensions intended to be secured flat onto the substrate of a printed circuit.
By reducing the dimensions, the thickness of the piezoelectric element can be as small as several tenths of a millimeter and must thus be protected from any external attack or contamination by being hermetically encapsulated in an enclosure, which may be placed in a protective atmosphere or a vacuum.
It is clear that this enclosure must nonetheless include external electric connection means for the excitation electrodes arranged on either side of the piezoelectric element, these connection means usually also forming means for securing the resonator to the substrate.
Since enclosures formed by glass bulbs provided with through connection pins, and metal capsules provided with ceramic pearls for connection pins or assemblies of the metal-ceramic type, have long since been abandoned, the most recent construction principle to which the present invention refers corresponds for example to the device described in U.S. Pat. No. 4,451,754. This device includes a rectangular plate of piezoelectric material cut out to form a frame-support to which is attached the resonant element situated at the center, the two faces being metallised to form both the excitation electrode and, for the frame, means for securing two symmetrical covers formed to leave a space free above the resonant element.
When they are made in a single part, the covers are stamped and each have a connection lug extending parallel to the piezoelectric plate, on either side of it. When they are made in two parts, each cover is formed of a metal frame and a rectangular closing plate, the connection lugs then being able to be formed with the same arrangements as before by extensions of the metal frame.
When the resonator which has just been briefly described, is arranged flat on a substrate, it can be seen that the connection lugs are in parallel planes which are different from the plane of said substrate, which has the drawback of preventing mounting the resonator on the substrate solely by means of these connection lugs. It is well known that a non negligible part of the vibrator power is transmitted to the substrate, which may over the long term have a detrimental effect on the quality and longevity of the resonator if it is not designed with efficient and preferably simple mounting onto the substrate.
The object of the present invention is to overcome the drawbacks of the aforecited prior art by providing an ultra-thin resonator whose electric connection means are in a same plane and are also used as means for mounting it on a printed circuit substrate.
The invention therefore concerns an encapsulated piezoelectric resonator including on the one hand a central plate made of a piezoelectric material cut to form a frame and a central resonant element connected to said frame, the top and bottom faces having a metal coating, on the other hand bottom and top covers, each being formed by a spacing frame made of metal or a metal alloy and a closing plate, characterised in that at least one of the elements forming the covers is electrically connected to the frame of the central plate and has an extension bent into a Z shape to bring its distal portion in line with a covering plate and to form electrical and mechanical connection means situated in a same plane.
The preferred embodiment is that in which all the portions constituting the covers are metal, the covering plate of the top cover then being the only one to have an extension bent into a Z shape. According to a variant, the bottom plate can also have a planar extension beyond the central plate, and thus allows the securing of the connecting lugs onto a printed circuit to be made easier for example by means of a laser beam.
According to another embodiment, the Z-shaped bent extensions are formed from at least one spacing frame. In the event that each spacing frame is provided with a Z extension, which will evidently be at different places on the periphery of the frame, the top and bottom covering plates could then be formed of an insulating material, such as a thin glass plate.
When the external parts of the covers are made of metal and have to be insulated, it is of course possible to apply an insulating varnish in accordance with known techniques.